1. Field of the Invention
The present invention relates generally motor control systems, and more specifically to motor control system that controls positioning motor torque.
2. Background of the Invention
Motor control systems are used in many applications, including industrial and consumer applications. The control of a motor's position, velocity and/or acceleration is the object of a motor control system, which attempts to position or control the speed of the motor at a commanded rotation, location and/or velocity.
FIG. 8 shows a motor drive control system as is known in the related art. The apparatus includes a power converter 25 such as an inverter for driving a 3-phase alternating-current (AC) motor 27, having a direct-current (DC) power supply 21 as an input, and a control device 100 for controlling the system.
Power converter 25 converts DC power from DC power supply 21 into AC power and supplies it to motor 27. A circuit breaker 23 interrupts the supplied power when there is a failure such as a short-circuit of power converter 25, interrupting the circuit so that excessive current does not flow from DC power supply 21 to power converter 25. A voltage detector 24, such as an instrument transformer detects the input voltage of power converter 25 and a current detector 26, such as a current transformer, detects the output current of power converter 25.
Control device 100, which supplies a drive signal to power converter 25 includes a speed calculating part 3 for determining a speed of motor 27 from a detected position value provided by an encoder 4 mechanically coupled to motor 27. The control device 100 also includes a torque command calculating part 2 for calculating a torque command value for motor 27 from the difference between a speed command value and the speed value determined by speed calculating part 3. An output calculating part 1 is provided for providing feedback control based on the difference between an output current command value for power converter 25 obtained from the torque command value and a detected output current value provided by the current detector 26. Output calculating part 1 then generates a drive signal provided to power converter 25 using the detected input voltage value from voltage detector 24 and the detected position value.
In the system depicted in FIG. 8, considering the input impedance 22 due to the resistance of the wiring between the DC power supply 21 and power converter 25 and other elements, the input voltage V1 and the output power P of power converter 25 are given by the following equations:V1=V0−R·I and P=I·V1=I(V0−R·I),respectively, where V0 is the output voltage of DC power supply 21, I is the input current of power converter 25; and R is the value of the input impedance 22.
The maximum output power of the system Pmax is then V02/4R and the values of the input voltage V1 and the input current I at that time are V1=V0/2 and I=V0/2R, respectively. From the above expressions it is apparent that input impedance R cannot be ignored, since if the input current is increased, at a certain point a maximum output power is reached. Even if the input current is increased beyond the maximum power point, output power P decreases due to a voltage drop caused by input impedance 22.
The specification for maximum output power level is set in consideration of the above constraints, but when the required output power is greater than the maximum output value Pmax at a given time, the required torque command value cannot be met by actual torque, and the difference between the output current command value obtained from the torque command value and the detected output current value widens. Consequently, if output-calculating part 1 integrates the difference, the integral term becomes large, and the current control output becomes saturated. As a result the responsiveness of the system falls.
In the related art shown in FIG. 8, when the maximum output value of the power converter 25 is reached, when an output demand such as for further motor acceleration is made, an overcurrent condition typically arises on the input side of power converter 25 and the circuit is broken by circuit breaker 23. The output of the power converter 25 ceases supplying power, and in an apparatus in which continuous operation of the motor 27 is required, the overload condition inevitably results in total system failure.
Known solutions exist to the above-described problem, as exemplified by a system as disclosed in Japanese Patent JP-A-2003-153575. In that system, a control device is included using current control to calculate an output voltage command value by integrating the difference between an output current command value and a detected output current value. The system stops the integral control when the output voltage command value has risen above a predetermined limit value, thereby preventing saturation of current control output accompanying output voltage limitation is disclosed.
However, the above-described solution has disadvantages, as using a predetermined limit value on the output voltage command value, as the current control output can become saturated when the power supply voltage falls. In that condition, the maximum output value of the power converter falls below the maximum output specification, and because the output voltage command value then never reaches the predetermined limit, again causing saturation of the control function and failure of responsiveness of the system.
A second solution provided in another electric motor drive control device wherein control is prevented from becoming unstable when the power supply voltage of a power converting device drops is set forth in Japanese Patent JP-A-2002-218799.
In the system described in the above-referenced Japanese Patent, when the power supply voltage drops and the output voltage of the power converting device becomes saturated, the condition is recognized and the electric motor current is lowered. Thus, the drop in the power supply voltage is suppressed and simultaneously the voltage across output demand lines is suppressed, thereby preventing the current feedback value from failing to follow the current command value.
The above-described second solution also has disadvantages. Although torque limitation during a drop in power supply voltage is possible, the effect of the voltage drop at the converted input that is caused by the input impedance is not considered. Again, as in the case of the related art shown in FIG. 8, continuous operation of the motor is not guaranteed.
Therefore, it would be desirable to provide a motor control system in which a voltage drop due to input impedance or due to a fall in power supply voltage has reduced effect on the responsiveness of the system. If is further desirable to provide a motor control system in which torque control is maintained via an upper limit on power converter output and in which motor operation can be continued without stopping.